1. Field of the Invention
The present invention relates to a motor control device.
2. Discussion of the Background
In order to detect the magnetic pole of the rotor in a motor, a typical control device makes use of the magnetic saturation. One example of such a motor control device is as follows.
U.S. Patent Application Publication No. 2002/0060548 discloses a motor control device, which applies a minute voltage change to a voltage command on a dc axis which constitutes an estimation magnetic pole axis of a motor. Then, this motor control device discriminates between polarities of the magnetic pole axes by making use of the difference between respective cycles in which ripple components of the electric currents are positive and negative, or the difference between respective current change rates on the positive and negative sides, in response to the applied minute voltage change. Further, the motor control device applies the minute voltage change to the above voltage commands on both of the dc axis and the qc axis that is orthogonal to the dc axis, and directly estimates the magnetic pole position of the motor, on the basis of the current ripple components on the respective axes.
The above-mentioned motor control device utilizes the magnetic saturation, namely, the difference between the inductances Ld on the N and S poles, and compares the difference between the conducting times of positive and negative motor currents generated in response to the minute voltage change, thereby detecting the polarity of the rotor magnetic pole.
However, the magnetic saturation property of motors is dependent on the magnetic materials used in the motors. In addition, neodymium magnets for industrial applications have a high magnetic operating point. Specifically, the magnetizing state of the magnetic circuit that is composed of the stator and the rotor in the motor is close to the saturation state of the hysteresis expressed by the B-H curve (or hysteresis loop), when no current flows through a motor. Therefore, it is difficult to monitor the influence of saturation by using such a minute voltage change.